Optical Receivers with Controllable Transfer Function Bandwidth and Gain Imbalance

ABSTRACT

Techniques, devices and systems based on optical receivers with controllable transfer function bandwidth and gain imbalance.

PRIORITY CLAIMS AND RELATED APPLICATIONS

This patent document claims the priority of U.S. Provisional ApplicationNo. 61/163,850 entitled “OPTICAL RECEIVERS WITH CONTROLLABLE TRANSFERFUNCTION BANDWIDTH AND GAIN IMBALANCE” and filed on Mar. 26, 2009. Thispatent document is also a continuation-in-part of U.S. patentapplication Ser. No. 11/726,557 entitled “and filed Mar. 22, 2007. Theabove referenced applications are incorporated by reference as part ofthe disclosure of this document.

BACKGROUND

This patent document relates to optical communications.

Optical communications use an optical modulator to modulate an opticalcarrier beam to carry digital bits for transmission over an opticallink. An optical communication system can use optical wavelengthdivision multiplexing (WDM) to transmit multiple optical carriersmodulated to carry different optical data channels through a singlefiber. The performance of optical transmission can be characterized byvarious parameters, such as the optical signal to noise ratio (OSNR),the data bit error rate (BER) and the data bit rate per wavelength ordata spectral efficiency. The signal quality of an optical WDM signalmay be degraded by various effects in the optical transmission such asoptical attenuation effects in fiber and optical dispersion effectsincluding chromatic dispersion (CD), polarization mode dispersion (PMD)and polarization dependent loss (PDL) in fiber. Various signalmodulation techniques and associated signal detection techniques can beused to generate modulated data formats that tolerate signal degradationeffects in optical transmission and thus improve the transmissionperformance.

SUMMARY

The techniques, devices and systems described in this document includeoptical receivers with controllable transfer function bandwidth and gainimbalance.

Implementations of the described techniques, devices and systems can usea suitable optical modulation format to produce an optical transmissionsignal that tolerates signal degradation effects in optical transmissionand uses an optical receiver that includes ea delay line interferometer(DLI), a reconfigurable free spectral range (FSR) phase controller, anda reconfigurable gain imbalancer controller to optimize the opticalreceiver in detecting the optical transmission signal. Examples ofsuitable optical modulator formats for implementing the describedtechniques, devices and systems include but are not limited to phaseshift keying (PSK) modulation formats, such as differential PSK (DPSK),Quadrature PSK(QPSK), differential QPSK, 8-PSK, and quadrature amplitudemodulation (QAM) formats.

In one aspect, an optical receiver includes an optical delay lineinterferometer comprising an optical input port that receives amodulated optical input signal carrying a baseband signal, a firstoptical path that receives a first part of the received modulatedoptical input signal, a second optical path that receives a second partof the received modulated optical input signal, and an optical outputport that combines light from the first and second optical paths tocause optical interference and produces a constructive optical outputsignal having a constructive transfer function and a destructive opticaloutput signal having a destructive transfer function. This receiver alsoincludes a first optical detector that converts the constructive opticaloutput signal into a first electrical signal; a second optical detectorthat converts the destructive optical output signal into a secondelectrical signal; a signal combiner that combines the first and secondelectrical signals to produce a difference between the first and secondelectrical signals that represents a recovery of the baseband signal inthe received modulated optical input signal; a data estimator thatprocesses the difference between the first and second electrical signalsto recover digital data bits from the difference produced by the signalcombiner; an optical control element disposed in at least one of thefirst and the second optical paths in the optical delay lineinterferometer to control a transfer function bandwidth of theconstructive transfer function and the destructive transfer function; afirst feedback circuit that receives information on a signal quality ofthe recovered digital data bits and produces a first feedback controlsignal to the optical control element to set the transfer functionbandwidth in response to the signal quality to enhance the signalquality; a signal gain imbalance control unit that controls a differencebetween amplitudes of the first and second electrical signals at thesignal combiner; and a second feedback circuit that receives theinformation on the signal quality of the recovered digital data bits andproduces a second feedback control signal to the signal gain imbalancecontrol unit in response to the signal quality to enhance the signalquality.

In another aspect, a method is provided for using an optical receiver toextract digital data from a modulated optical input signal. This methodincludes operating an optical delay line interferometer in an opticalreceiver to convert the modulated optical input signal based on opticalinterference into a constructive optical output signal and a destructiveoptical output signal; operating a first optical detector in the opticalreceiver to convert the constructive optical output signal into a firstelectrical signal; operating a second optical detector in the opticalreceiver to convert the destructive optical output signal into a secondelectrical signal; using a difference between the first and secondelectrical signals to recover a baseband signal carried by the receivedmodulated optical input signal; measuring a signal quality of therecovered baseband signal; controlling the optical delay lineinterferometer to control a transfer function bandwidth of theconstructive and destructive optical output signals to improve themeasured signal quality; and controlling an imbalance between amplitudesof the first and second electrical signals to improve the measuredsignal quality.

In another aspect, a method is provided for using an optical receiver toextract digital data from a modulated optical input signal. This methodincludes operating an optical delay line interferometer in an opticalreceiver to convert the modulated optical input signal based on opticalinterference into a constructive optical output signal and a destructiveoptical output signal; operating a first optical detector in the opticalreceiver to convert the constructive optical output signal into a firstelectrical signal; operating a second optical detector in the opticalreceiver to convert the destructive optical output signal into a secondelectrical signal; producing a difference between the first and secondelectrical signals; operating a data estimator to process the differenceto recover data of a baseband signal carried by the received modulatedoptical input signal; and using a threshold used by the data estimatorfor recovering the data as an indicator of the signal quality to controlan imbalance between amplitudes of the first and second electricalsignals to set the threshold to zero during normal operation of theoptical receiver.

In another aspect, an optical receiver is described in this document toinclude a signal processor having constructive and destructive transferfunctions for receiving and demodulating an optical signal havingdifferential modulation. The signal processor can be implemented toinclude a delay line interferometer (DLI), a free spectral range (FSR)phase controller, and a gain imbalancer. The DLI has a transit timedifference Y between two signal paths for demodulating the differentialmodulation signal and defining a free spectral range (FSR) bandwidth ofconstructive and destructive transfer functions. The FSR is calculatedor adjusted so that the performance benefit obtained by controlling thetransfer functions for reducing ISI distortion is greater than theperformance that is lost by not maximizing the demodulated signals atconstructive and destructive outputs when the time difference Y is notequal to the symbol time of the modulated signal. The FSR phasecontroller adjusts the phases of the constructive and destructivetransfer functions to tune the FSR transfer functions relative to thecarrier of the modulated optical signal. The gain imbalancer applies acalculated or adjusted unequal gain to the signals in the constructiveand destructive paths for determining or modifying the constructive anddestructive transfer functions.

In another aspect, an optical receiver is described to include a signalprocessor having constructive and destructive transfer functions forreceiving a modulated optical input signal and issuing signals atconstructive and destructive outputs, respectively; at least onetransfer phase element disposed in the signal processor, the transferphase element for providing a controllable transfer function phase forat least one of the transfer functions with respect to a frequency ofthe input signal; and a transfer phase controller coupled to thetransfer phase element for controlling the transfer function phase formaximizing a difference between signal powers for the constructive anddestructive outputs.

In another aspect, a method is provided for receiving an optical signaland includes applying constructive and destructive transfer functions toa modulated optical input signal for providing signals at constructiveand destructive outputs, respectively, at least one of the transferfunctions having a controllable transfer function phase; and controllingthe transfer function phase with respect to a frequency of the opticalsignal for maximizing a difference between signal powers for theconstructive and destructive outputs.

In another aspect, an optical receiver is provided to include a signalprocessor having constructive and destructive transfer functions forreceiving a modulated optical input signal and issuing signals atconstructive and destructive outputs, respectively; and a transferbandwidth element disposed in the signal processor for controlling atransfer function bandwidth for at least one of said constructive anddestructive transfer functions, the transfer function bandwidth selectedfor compensating for intersymbol interference in the input signal.

In another aspect, a method is provided for receiving a modulatedoptical signal and includes applying constructive and destructivetransfer functions, at least one of said transfer functions having aselected transfer function bandwidth, for receiving a modulated opticalinput signal and issuing signals at constructive and destructiveoutputs, respectively; and selecting said transfer function bandwidthfor compensating for intersymbol interference in said input signal.

In another aspect, an optical receiver is provided for receiving amodulated optical signal to include a signal processor for separating amodulated optical input signal into constructive and destructive signalpaths; and an optical gain imbalancer disposed in at least one of thesignal paths for selecting an optical gain imbalance between the signalpaths based on an effective bandwidth of the input signal forcompensating for signal impairments in the input signal.

In another aspect, a method is provided for receiving a modulatedoptical signal and includes separating a modulated optical input signalinto optical constructive and destructive signal paths; and selecting anoptical gain imbalance between the signal paths based on an effectivebandwidth of the input signal for compensating for signal impairments inthe input signal.

These and other aspects and their implementations are described indetail in the drawings, the detailed description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vector diagram of a BPSK signal;

FIG. 2 is a chart of constructive and destructive transfer functions ina delay line interferometer (DLI) for an adjustable free spectral range(FSR);

FIG. 3 is a block diagram of an exemplary optical transmission systemfor receiving a modulated optical signal;

FIG. 4 is a general block diagram of an optical receiver for the systemof FIG. 3;

FIG. 5 is a detailed block diagram of an optical receiver including adelay line interferometer (DLI) for the system of FIG. 3;

FIGS. 6A, 6B and 6C illustrate delay line interferometers (DLI)s for thereceivers of FIG. 5;

FIG. 7 is a simplified flow chart of a method for receiving a modulatedoptical signal;

FIG. 8 is a flow chart of a method using a calculated FSR and acalculated gain imbalance;

FIG. 9 is a flow chart of a method where the FSR and the gain imbalanceare adjusted for best signal quality;

FIG. 10 is a chart showing a calculation of FSR based on systembandwidth in order to compensate for the ISI in the system of FIG. 3;

FIG. 11 is a chart showing a calculation of gain imbalance based onsystem bandwidth and FSR in order to compensate for the ISI in thesystem of FIG. 3; and

FIGS. 12-16 illustrate examples of imbalance control operations.

DETAILED DESCRIPTION

For an optical system with filters, the effective concatenated bandwidthof the filters induces intersymbol interference (ISI). The ISI causesdistortion of the signal and reduces the decision quality (the abilityto accurately detect if a bit is a logical “1” or “0”) at a receiver.This decision quality may be quantified by counting the number of errorbits and dividing it by the total number of transmitted bits. Theresulting ratio is called bit error ratio (BER). Another way ofdiscussing the quality of the signal at the receiver involvestranslating the BER to a parameter called Q using the equation whereerfc-1 is the inverse complementary error function. The distortioneffect of ISI on signal quality may be viewed in a general way in abaseband eye diagram of the modulated signal where ISI causes the spacebetween “1” and “0” symbol levels to be partially filled by the trailingand leading edges of the symbols.

Optical signals commonly use binary phase shift keyed (BPSK) modulationwhere a carrier is modulated for data bits for logical “0” and “1” withphase shifts of 0 and π radians. The logical “0” or “1” is decoded atthe receiver by determining whether the detected signal is to the leftor right of a vertical imaginary axis of a signal vector diagram,sometimes called an IQ diagram. A detector viewed as a polar detectordetermines whether the absolute value of the received phase is greaterthan π/2 for “0” and less than π/2 for “1”. A detector viewed as arectangular detector determines whether the cosine of the phase of thesignal is negative or positive for “0” or “1”.

The BPSK optical signals may use a differentially-encoded phase shiftkeyed (DeBPSK, or DPSK) modulation format. The DPSK modulation formatencodes input data as the difference between two consecutive transmittedsymbols. The input data is differentially pre-coded using the precedingsymbol as a reference with an electrical “delay+add” function so that aninput data bit of logical “0” or “1” is encoded as a change of carrierphase of 0 or π radians relative to the preceding bit. At the detectorthe process is reversed by comparing a current bit to the preceding bit.

The DPSK decoding function may be performed using a delay lineinterferometer (DLI) and a balanced detector. The interferometer workson the principle that two waves that coincide with the same phase willadd to each other while two waves that have opposite phases will tend tocancel each other. The interferometer has an input port for receivingthe optical signal and two output ports—a constructive output port forissuing the waves that add and a destructive output for issuing thewaves that tend to cancel.

The delay line interferometer (DLI) for DPSK signals has an additionalelement of an internal delay difference between the two waves that isabout equal to the symbol time T of the DPSK modulation. Theconstructive output port issues a signal Ec=E(t)+E(t-T) and thedestructive output port issues a signal Ed=E(t)−E(t-T). The effect ofthe time T is to reverse the signals at the two output ports so that thewaves add at the destructive output port and cancel at the constructiveoutput port when consecutive bits differ by π radians. The differencebetween Ec and Ed can be detected with a direct detection intensityreceiver to determine when there is a change in phase in the signalbetween two consecutive bits and thereby estimate the logical bitscarried by the DPSK modulation.

It is an effect of this delay difference to impose a transfer functionhaving a sinusoidal amplitude response (in the frequency domain) fromthe input port to each output port. The spectral period of a cycle ofthe transfer function, equal to 1/T, is termed the free spectral range(FSR). The sinusoidal width proportional to the FSR effectively limitsthe frequency band of the signals that can be passed from the DLI inputto the constructive and destructive outputs. The phase of the frequencydomain cycle of the transfer function is termed the FSR phase.

It is commonly believed that a DLI delay difference equal to the symboltime T, and an FSR equal to the inverse of the symbol time T, is desiredin order to provide the best system performance (fewest data estimationerrors) by maximizing the difference between the signals Ec and Ed atthe constructive and destructive outputs. Considered by itself, adifferential delay not equal to the symbol time T would be expected todegrade system performance because the current and preceding symbols arenot exactly differentially compared.

The examples in this document are described in terms of binary phaseshift keyed (BPSK) signals using a differentially-encoded BPSK (DeBPSK,or DPSK) modulation format. However, the techniques, devices and systemsdescribed in this document may be carried out with various PSKmodulation formats including higher order modulation formats such asquadrature phase shift keyed (QPSK), 4QAM, 8PSK, 16QAM and so on. Forexample, differentially-encoded QPSK (DQPSK) can be used.

FIG. 1 is a vector representation of a binary phase shift keyed (BPSK)optical signal having phase states of 0 and it radians. Real (in-phaseor “I”) and imaginary (quadrature phase or “Q”) parts of the complexBPSK optical signal are shown on horizontal and vertical axes,respectively. The BPSK signal between phase states of 0 and it may havea trajectory in the IQ plane of pure phase modulation (continuouslychanging phase with constant amplitude); or a trajectory in the IQ planeof Mach-Zehnder modulation (continuously changing amplitude through anamplitude null); or anything in between. For a DPSK modulation format,the logical bits are encoded as the differences between consecutivephase states.

FIG. 2 is a chart showing exemplary constructive and destructivetransfer functions, referred to below as G(f) and H(f), between an inputport and constructive and destructive output ports for a signalprocessor having a delay line interferometer (DLI). The transferfunctions G(f) and H(f) are frequency responses of transmitted opticalpower versus frequency. The vertical axis of the chart shows powertransmission. The horizontal axis of the chart shows frequency for anoptical input signal scaled to modulation symbol rate R, relative to acenter frequency of the transfer functions. The center frequency of thetransfer functions is shown as zero. The scale factor R is the inverseof the symbol time T for modulation phase states carried by the opticalsignal.

The DLI has a transit time difference Y for demodulating adifferentially modulated signal. The transit time difference Y (FIGS. 4and 5) is also referred to in some places as the differential transittime Y or simply as the time Y. The inverse of the time Y is the freespectral range (FSR) of the DLI. Looked at another way, the FSR of theDLI is defined as the period of the transfer functions G(f) and H(f).The constructive and destructive transfer functions G(f) and H(f) areshown for free spectral ranges (FSR)s of 1.0R, 1.1R, 1.2R and 1.3R.Increasing the FSR effectively increases the bandwidth of theconstructive and destructive transfer functions. The bandwidth of theconstructive transfer function in this case is the frequency spectrumbetween points at one-half the maximum amplitude or where theconstructive and destructive transfer functions cross. The bandwidth ofthe destructive transfer function is understood to be the bandwidth ofthe stop band of the constructive transfer function or where theconstructive and destructive transfer functions cross. Equations 1 and 2show constructive and destructive transfer functions G(f) and H(f),respectively, for the DLI.

G(f)=[1+cos (2πfY)]/2   (1)

H(f)=[1−cos (2πfY)]/2   (2)

It can be seen that the FSR transfer functions G(f) and H(f) areperiodic in the frequency domain. Phase of the periodic transferfunction (offset in the frequency domain) is known as an FSR phase. Inan optical system using differential modulation, best signal quality maybe obtained when the FSR phase is adjusted so that the transferfunctions G(f) and H(f) have a maximum ratio or normalized difference(difference scaled by the sum) at the carrier frequency of the opticalsignal or the center of the energy in the spectrum of the modulatedoptical signal. FIG. 2 shows the correct adjustment for the transferfunction phase or FSR phase for maximum transfer function differencewith the center frequency of the transfer functions aligned to thecenter frequency and carrier frequency of the received optical inputsignal for a symmetrical optical input signal spectrum.

FIG. 3 is a block diagram of an exemplary data transmission systemreferred to with a reference number 10. The system 10 includes anoptical transmitter 12 and an optical receiver 20. The transmitter 12and the receiver 20 are connected through an optical transmission link16. The transmission link 16 may use wavelength division multiplexing(WDM) for carrying several optical signals simultaneously usingdifferent optical carrier frequencies.

The transmitter 12 transmits an optical signal using adifferentially-encoded phase shift keyed (DPSK) modulation format wherelogical 1's and 0's of input data are encoded as phase differencesbetween adjacent (consecutive in time) phase states. For example forDPSK, adjacent phase states of 0 radians and adjacent phase states of itradians both carry a data bit having a logical “0”; and a phase state of0 radians following a phase state of it radians and a phase state of πradians following a phase state of 0 radians both carry a data bithaving a logical “1”. Of course, the logical “0” and logical “1” may bereversed without loss of generality. It should also be noted that anytwo phase states that are separated by π radians may be used for theDPSK modulation.

The transmitter 12 illuminates one end of the link 16 with a modulatedoptical signal 22 having differentially-encoded phase shift keyed (DPSK)modulation for the logical bits of input data. The signal 22 passesthrough the link 16 and emerges at the other end of the link 16 as amodulated optical signal 24 to be received by the receiver 20. The link16 has a frequency response having an effective optical bandwidth causedby one or more filters represented by filters 26. The optical bandwidthof the link 16 results in an effective optical bandwidth of the spectrumof the input signal 24.

The receiver 20 demodulates the signal 24 for providing output data thatis its best estimate of the input data. The output data is desired to bean exact replica of the input data. However, the transmission link 16degrades or impairs the quality of the received signal 24 and thisdegradation or impairment in signal quality causes the receiver 20 tooccasionally make errors in the output data that it provides. One of theprimary causes of the signal degradation is intersymbol interference(ISI) in the link 16 induced by the filters 26. The receiver 20 hasapparatus and methods, described below, for compensating for the qualitydegradation in the link 16, especially the ISI, in order to reduce theerrors in the output data.

The apparatus and methods of the receiver 20 use measurements of signalquality and calculations based on the effective optical bandwidth of thelink 16 and/or the effective optical bandwidth of the input signal 24for compensating for one or more signal degradations or impairments inthe input signal that may include but are not limited to ISI,signal-dependent noise and signal independent noise. The signal qualitymeasurements may be bit error ratio (BER) measurements or eye openingratio measurements. In some cases the signal quality measurements mayuse signal-to-noise measurements taken from optical or electricalconstructive and destructive path signals in the receiver 20. In someimplementations, the receiver 20 uses calculations based on theeffective optical bandwidth of the link 16 for minimizing the BER forthe received input signal 24.

FIG. 4 is a block diagram of an optical receiver referred to with thereference number 20. The receiver 20 receives the optical signal 24 andprovides output data that is its best estimate of the input data thatwas transmitted by the transmitter 12.

The receiver 20 includes a demodulator 30 and a data estimator 32. Thereceiver 20 or an external computer includes a bandwidth controlalgorithm 33. The demodulator 30 demodulates the optical input signal 24and issues an electrical baseband signal. The data estimator 32processes the baseband signal and issues the output data. The receiver20 may also include an input optical filter for filtering the opticalsignal 24 into a channel when the optical signal 24 is wavelengthdivision multiplexed (WDM) and contains multiple channels.

The demodulator 30 includes a signal processor 34, a detector apparatus35, a combiner 36, and a transfer phase controller 37. The signalprocessor 34 has two parts, an optical signal processor 34A and anelectrical signal processor 34B. The optical signal processor 34Areceives the signal 24 at an input port 42; separates the signal 24 intooptical constructive and destructive interference signals;differentially demodulates the signal 24 with a differential transittime Y; and issues the signals at constructive and destructive outputports 43A and 44A, respectively. The detector apparatus 35 receives theoptical constructive and destructive paths signals from the ports 43Aand 44A and converts photons to electrons for providing electricalconstructive and destructive path signals shown as electrical currentsi_(G) and i_(H) for the modulations on the optical signals.

The signal processor 34B processes the electrical signals and passes theprocessed electrical signals through constructive and destructive outputports 43B and 44B, respectively, to the combiner 36. The combiner 36takes a difference between the instantaneous signal level of theconstructive path signal and the instantaneous signal level of thedestructive path signal for providing the baseband signal. In avariation of the receiver 20, the data estimator 32 connects to theports 43B and 44B for receiving differential electrical signals.

The separation of the input signal 24 using optical interference intothe constructive and destructive paths provides the constructive anddestructive transfer functions G(f) and H(f), respectively, in thesignal processor 34A. The transfer functions G(f) and H(f) are a part ofthe constructive and destructive transfer functions provided by thesignal processor 34 and the detector apparatus 35 from the input port 42to the constructive and destructive output ports 43B and 44B,respectively. In some implementations, the constructive and destructivetransfer functions can be primarily determined within the signalprocessor 34A to the output ports 43A and 44A.

The transfer phase controller 37 includes a detector 45 for measuringand averaging power-related levels for the signals at the output ports43A and 44A (or 43B and 44B). The power-related levels that are measuredare indicative of, or have a monotonic relationship to, the signalpowers at the output ports 43A and 44A (or the output ports 43B and44B). For example, the measurements may be signal power, average signalmagnitude, squared signal level, or absolute value of signal level withan arbitrary exponent. The transfer phase controller 37 uses themeasurements for providing a feedback signal that maximizes the ratio ofthe signal power for the port 43A to the signal power for the port 44A(or the signal power for the port 43B to the signal power for the port44B). The idea may also be used in an inverted mode for maximizing theratio of the signal power for the port 44A to the signal power for theport 43A (or the signal power for the port 44B to the signal power forthe port 43B).

The signal processor 34A has controllable transfer phase elements 46Gand 46H for providing adjustable phase shifts Φ_(G) and Φ_(H) for theconstructive and destructive transfer functions. The elements 46G and46H may be the same physical element 46 and the phase shifts Φ_(G) andΦ_(H) may be the same phase shift Φ. The transfer phase controller 37uses the power-related measurements from the detection 45 forcontrolling the elements 46G and 46H, or the element 46, for adjustingthe phases Φ_(G) and Φ_(H), or the phase Φ, for shifting the phases ofthe transfer functions for a maximum normalized signal power differencebetween the signals at the constructive port 43A (or 43B) and thedestructive port 44A (or 44B). This process may be used to tune thetransfer functions G(f) and H(f) relative to the carrier frequency ofthe modulated optical signal 24 and at the center frequency of theenergy in the modulated optical signal 24.

The signal processor 34A has a transfer bandwidth element 48 forproviding a selectable or controllable bandwidth (BW). At least one ofthe constructive and destructive transfer functions depends, at least inpart, upon this bandwidth. In some implementations, the optical signalprocessor 34A includes a delay line interferometer (DLI). In this casethe bandwidth is defined or modified by the inverse of the time Y.

During the design or installation of the receiver 20, or when thereceiver 20 is in operation, a calculation or test is made, or activefeedback is provided, for signal quality or a bit error ratio of theoutput data. A primary degradation of the signal quality in the system10 is intersymbol interference (ISI) caused by the filters 26. Thebandwidth control algorithm 33 calculates or provides feedback fordetermining or controlling the transfer bandwidth element 48 as shown inthe chart of FIG. 10. The calculation or test, or active feedback, isused for selecting or controlling the element 48 in order to select oradjust the bandwidth for providing the best signal quality or minimumISI for the system 10. The signal quality may be measured on the opticalor electrical signals, by measuring eye opening in a baseband signal orby measuring bit error ratio (BER).

An imbalance control algorithm 64 may be included for calculating a gainimbalance or providing feedback from signal quality data to the signalprocessor 34 to either the optical processor 34A or the electricalprocessor 34B or both for optimizing signal quality. The signalprocessor 34 uses the gain imbalance calculations or feedback toimbalance the gains between the constructive and destructive pathsignals. The gain imbalance calculations may be based on the effectiveoptical bandwidth of the link 16 and the input signal 24.

A side effect of changing the selection of the transit time difference Yis that the transfer function phase or FSR phase of the transferfunctions G(f) and H(f) may slide many cycles with respect to thefrequency of the input signal 24. In a general rule, whenever the FSRdelay is changed, the transfer function phase shift Φ, or phase shiftsΦ_(G) and Φ_(H), must be re-adjusted by the transfer (FSR) phasecontroller 37 by adjusting the transfer (FSR) phase element 46, or 46Gand 46H, for re-centering the transfer functions G(f) and H(f) to itsoptimal frequency position. When the received optical spectrum issymmetrical, the optimal position coincides with the carrier frequencyof the input optical signal 24. On the other hand the effect of changingthe phase shift Φ, or phase shifts Φ_(G) and Φ_(H), on the FSR bandwidthis so small that is insignificant.

The receiver 20 may also include a path for signal quality feedback 92.Data for signal quality is processed through the signal quality feedback92 and passed to the transfer phase controller 37. The transfer phasecontroller 37 uses the processed signal quality data for fine tuning thephase delay of the transfer phase element 46 for improving andoptimizing the signal quality. Preferably, the element 46 is first tunedin a feedback loop according to the power-related measurements and thenfine tuned in a second feedback loop for minimizing a bit error ratio(BER). The signal quality data may be obtained by measuring BERdirectly, by measuring an eye opening ratio of a baseband signal, and/orby measuring a signal to noise ratio (SNR) of the optical or electricalconstructive and destructive path signals.

FIG. 5 is a detailed block diagram of an optical receiver referred towith a reference number 120. The receiver 120 is an embodiment of thereceiver 20 described above for the system 10. Elements of the receiver120 that are analogous to, or embodiments of, elements of the receiver20 are denoted by incrementing the reference identification numbers by100.

The receiver 120 includes a demodulator 130, a data estimator 132 and abit error ratio (BER) detector 138. The receiver 120, or an externalcomputer, also includes a bandwidth (FSR) control algorithm 133, and animbalance control algorithm 164. The demodulator 130 demodulates theoptical signal 24 and passes the demodulated electrical signal to thedata estimator 132. The data estimator 132 processes the electricalsignal for making a best estimate of the original input data and issuesits best estimated input data as output data. The BER detector 138estimates a BER for the output data. The BER may be used as signalquality data. The demodulator 130 uses the signal quality data throughthe algorithms 133, 164 and 192.

The demodulator 130 includes a signal processor 134, a detectorapparatus 135, a combiner 136 and a transfer free spectral range (FSR)phase controller 137. The signal processor 134 includes an opticalsignal processor 134A and an electrical signal processor 134B. Theoptical signal processor 134A receives the optical input signal 24 at aninput signal port 142; separates the signal 24 into optical constructiveand destructive interference signals; differentially demodulates thesignal 24 with the differential time Y; and issues signals fromconstructive and destructive output ports 143A and 144A, respectively,to the detector apparatus 135.

The detector apparatus 135 converts the modulations on the opticalconstructive and destructive path signals to electrical signals andpasses the electrical signals to the electrical signal processor 134B.The electrical signal processor 134B processes the electrical signalsand issues the processed electrical signals at constructive anddestructive output ports 143B and 144B, respectively, to the combiner136. The combiner 136 takes a difference between the instantaneoussignal level of the constructive path signal and the instantaneoussignal level of the destructive path signal for providing the basebandsignal. In a variation of the receiver 120, the data estimator 132connects to the ports 143B and 144B for receiving differentialelectrical signals.

The optical signal processor 134A includes a delay line interferometer(DLI) 150 and an optical imbalancer 152. The electrical signal processor134B includes an electrical imbalancer 156. The DLI 150 has an inputport 165 connected to the input port 142 of the demodulator 130 forreceiving the signal 24. The constructive transfer function of the DLI150 between the input port 165 and its constructive output port 166includes the transfer function G(f) of the equation 1. The destructivetransfer function of the DLI 150 between the input port 165 and itsdestructive output port 168 includes the transfer function H(f) of theequation 2.

The constructive transfer function of the signal processor 134 betweenthe input port 142 and the constructive output port 143B includes theconstructive transfer function of the DLI 150 and the transfer functionsin the constructive signal path of the optical imbalancer 152, thedetector apparatus 135 and the electrical imbalancer 156. Similarly, thedestructive transfer function of the signal processor 134 between theinput port 142 and the destructive output port 144B includes thedestructive transfer function of the DLI 150 and the transfer functionsin the destructive signal path of the optical imbalancer 152, thedetector apparatus 135 and the electrical imbalancer 156.

The signals at the constructive and destructive output ports 166 and 168may be created with optical interference by separating the input signalat the port 165 into two paths and then recombining the signals. The DLI150 has a first signal delay element referred to as a transfer freespectral range (FSR) bandwidth element 148 and a second signal delayelement referred to as a transfer (FSR) phase element 146. The FSR phaseelement 146 provides a delay difference between the signal transit timesin the signal paths in the DLI 150 and also provides a transfer functionphase shift Φ to the constructive and destructive free spectral rangetransfer functions for the DLI 150. The FSR bandwidth element 148provides a signal delay Z (FIGS. 6A-C) between the signal transit timesin the signal paths in the DLI 150.

The signal delay Z provided by the FSR bandwidth element 148 is calledan FSR delay to distinguish it from the signal delay difference providedby the FSR phase element 146 called an FSR phase delay. The readershould be aware that two different types of phases are being describedhere—the phases of the periodic signals and the phases of the periodictransfer functions G(f) and H(f). The FSR delay Z is a major contributorto the signal transit time difference Y for differentially demodulatingthe input signal 24. It should be noted that for the receiver 120, thetime difference Y will not, in general, be the same as the symbol time Tof the modulated signal 24. In a typical system 10, the time differenceY of the receiver 120 is less than about 83% of the symbol time T.

The inverse of the time difference Y defines the free spectral range(FSR) and the bandwidth of the constructive and destructive transferfunctions of the DLI 150. The free spectral range of the DLI 150determines or is a contributor to the constructive and destructivetransfer functions G(f) and H(f) for the DLI 150. The FSR delay Z of theFSR bandwidth element 148 is selected or adjusted based on known ormeasured characteristics of the link 16 to provide the time difference Ythat provides a desired free spectral range (FSR) for the DLI 150 forimproving the performance of the system 10, and especially for reducingthe signal quality degradation due to intersymbol interference (ISI)caused by the filters 26. The bandwidth (FSR) control algorithm 133calculates or provides feedback for determining or controlling theelement 148 as shown in the chart of FIG. 10. In some embodiments theFSR bandwidth element 148 and the FSR phase element 146 may be combinedas a single element having a large delay Z having a small adjustablerange for providing the phase shift Φ.

The FSR phase element 146 is used for fine tuning the phase Φ of thecyclic frequency response of the transfer functions G(f) and H(f) totune the transfer functions G(f) and H(f) relative to the carrierfrequency of the modulated input signal 24. In general, the FSR phasemust be re-adjusted each time a new FSR delay Z is selected or adjusted.The FSR phase element 146 may be controlled by a mechanism 174 includedin the DLI 150 where the mechanism 174 is controlled by the FSR phasecontroller 137. The mechanism 174 may be an oven for controlling thetemperature of the element 146.

The receiver 120 may include an input optical filter for filtering theoptical signal 24 into a channel when the optical signal 24 has multiplechannels that are wavelength division multiplexed (WDM). The inputoptical filter may be viewed as one of the filters 26 in the link 16. Itis desirable for cost and convenience that the same processor 134, andthe same DLI 150 be used for any channel.

In an exception to the general rule stated above, the FSR phasecontroller 137 and FSR phase element 146 may not be necessary when theFSR bandwidth element 148 is selected for providing the time differenceY exactly equal to the inverse of the frequency spacing of the channels.For example, for a channel spacing of 50 GHz and a symbol time of 23picoseconds, the time difference Y might be 20 picoseconds. However, inthis special case, the FSR of the DLI 150 may not be optimized for bestBER. In the receiver 120, the FSR bandwidth element 148 is selectedaccording to a criteria of compensating for ISI in the transmission link16 for providing the transit time difference Y and the FSR for best BERwhere the time difference Y is not the inverse of the channel spacing.

The optical imbalancer 152 includes constructive and destructivevariable gain elements 176 and 178 for controlling the optical gainsthat are applied to the signals from the output ports 166 and 168,respectively, in order to provide a gain imbalance between theconstructive and destructive signals to the output ports 143A and 144A.The gains of the elements 176 and 178 may be controlled by the imbalancecontrol algorithm 164 for varying the ratio of the power gains forconstructive and destructive paths for providing constructive anddestructive transfer functions g_(o)(f) and h_(o)(f) according torespective equations 3 and 4. In the equations 3 and 4, the optical gainimbalance, shown with symbol β_(o), varies from minus one to plus one.

g _(o)(f)=1−β_(o)   (3)

h _(o)(f)=1+β_(o)   (4)

The imbalance operation may be provided dynamically in a closed loopusing active feedback for minimizing the BER from the BER detector 138.Or, the imbalance operation may be “set and forget” (until it is set andforgotten again) after measuring the BER. Or, the imbalance operationmay be open loop based on calculations from known or measuredcharacteristics of the link 16. The calculations are shown in a FIG. 11that is described below. The gain elements 176 and 178 may use variableamplification or variable attenuation for providing the gain ratio. Onlyone of the gain elements 176 and 178 is required to be variable in orderto provide the variable gain ratio.

The detector apparatus 135 includes a constructive photo-detector 182and a destructive photo-detector 184 for detecting the optical signalsfor the ports 143A and 144A, respectively, by converting photons toelectrons for providing electrical signals to the electrical imbalancer156. Photodiodes may be used for the detectors 182 and 184. Eachphotodiode 182 and 184 produces an electrical signal proportional todetected optical power. The constructive and destructive transferfunctions from the input port 165 to the electrical outputs of thedetector apparatus 135 include the terms of respective equations 5 and6.

G(f)*g _(o)(f)={[1+cos (2πfY)]/2}*(1−β_(o))   (b 5)

H(f)*h _(o)(f)={[1−cos (2πfY)]/2)}*(1+β_(o))   (6)

The FSR phase controller 137 controls the phase delay of the FSR phaseelement 146 for maximizing a ratio of the optical powers in theconstructive and destructive detectors 182 and 184. In someimplementations, FSR phase controller 137 includes a detector 145 formaking a power-related measurement for the signals in the constructiveand destructive signal paths. The detector 145 measures and thenaverages the optical powers in the constructive and destructivedetectors 182 and 184 by measuring photocurrents A_(C) and A_(D),respectively. The photocurrents are the electrical currents in thedetectors 182 and 184 that result from the conversions of photons toelectrons. The photocurrents are measured by measuring the electricalcurrents passing through the detectors 182 and 184 and then averaging toremove high frequency components. The high frequency components can beremoved with low pass electrical filters with passbands lower than thebandwidth of the optical modulation.

An algorithm in the FSR phase controller 137 controls the phase delay ofthe FSR phase element 146 in order to maximize a ratio, difference ornormalized difference of the transfer functions. The normalizeddifference is the difference between the constructive and destructivesignal path power-related measurements divided by the sum of theconstructive and destructive signal path power-related measurement. TheFSR phase controller 137 may be constructed in order to maximize thenormalized difference ΔB measured from the average photocurrents asshown in an equation 7.

ΔB=(A _(C) −A _(D))/(A _(C) +A _(D))   (7)

The receiver 120 may also include a path for signal quality feedback192. Data for signal quality is processed through the signal qualityfeedback 192 and passed to the FSR phase controller 137. The FSR phasecontroller 137 uses the processed signal quality data for fine tuningthe phase delay of the FSR phase element 146 in order to improve andoptimize the signal quality. Preferably, the FSR phase element 146 isfirst tuned in a feedback loop for maximizing a constructive—destructivenormalized power difference and then fine tuned for minimizing a biterror ratio (BER). The signal quality data may be obtained by measuringBER directly, by measuring an eye opening ratio of a baseband signaland/or by measuring a signal to noise ratio (SNR) of the optical orelectrical constructive and destructive path signals.

The electrical imbalancer 156 includes constructive and destructivevariable gain elements 186 and 188 for controlling the electrical gainsapplied to the signals from the constructive and destructive detectors182 and 184, respectively, and issuing signals from output ports 143Band 144B. The gains of the elements 186 and 188 may be controlled by theimbalance control algorithm 164 for varying the ratio of the gains forconstructive and destructive paths for providing constructive anddestructive transfer functions g_(e)(f) and h_(e)(f) according torespective equations 8 and 9. In the equations 8 and 9, the electricalgain imbalance, shown with symbol β_(e), varies from minus one to plusone.

g _(e)(f)=1−β_(e)   (8)

h _(e)(f)=1+β_(e)   (9)

The imbalance operation may be provided dynamically in a closed loopusing active feedback for minimizing the BER from the BER detector 138.Or, the imbalance operation may be “set and forget” (until it is set andforgotten again) after measuring the BER. Or, the imbalance operationmay be open loop provided based on calculations from known or measuredcharacteristics of the link 16. The calculations are shown in a FIG. 11that is described below. The gain elements 186 and 188 may use variableamplification or variable attenuation for providing the gain ratio. Onlyone of the gain elements 186 and 188 is required to be variable in orderto provide the variable gain ratio.

The combiner 136 takes the difference between the electrical signalsfrom the constructive and destructive output ports 143B and 144B andpasses the difference as a baseband signal to the data estimator 132.The baseband signal is the demodulated signal corresponding to the inputsignal 24.

The baseband signal has instantaneous signal levels that in a systemwith no degradation would be exactly representative of the input data atsample times synchronized to a data clock. For example at the sampletimes, one signal level would represent a logical “1” and another signallevel would represent a logical “0” for the input data. However, varioussignal degradations, especially intersymbol interference (ISI) due tothe filters 26 in the link 16, cause the signal levels of the basebandsignal at the sample times to have many levels and occasionally evenhave levels where a “1” appears to be a “0” and vice versa. The basebandsignal synchronized to the data clock and shown over and over again onthe same display appears as an eye diagram where the opening of the eyeis a measure of the quality of the demodulated signal.

The data estimator 132 recovers frame and data clock signals and useserror detection and correction techniques for making its best estimateof the input data. Its best estimate of the input data is issued asoutput data. The BER detector 138 uses error detection and correctioninformation from the date estimator 132 and/or programmed knowledge ofexpected data bits in the output data for estimating a bit error ratio(BER). For dynamic operation, the BER detector 138 passes the BER to theimbalance control algorithm 164 in the demodulator 130. The function ofthe BER detector 138 for providing BER measurements or feedback may bereplaced or augmented with a device for measuring the signal quality ofthe baseband signal. The signal quality device and/or measurement may beinternal to the receiver 120 or external. Test equipment may be used asan external device for measuring signal quality or BER.

A side effect of changing the selection of the FSR delay Z is that thetransfer function phase or FSR phase of the transfer functions G(f) andH(f) may slide many cycles with respect to the frequency of the inputsignal 24. In a general rule, whenever the FSR delay is changed, thetransfer function phase shift Φ, or phase shifts Φ_(G) and Φ_(H), mustbe re-adjusted by the transfer (FSR) phase controller 137 by adjustingthe FSR phase element 146 for re-tuning the transfer functions G(f) andH(f) to the frequency of the input optical signal 24. On the other handthe effect of changing the phase shift Φ, or phase shifts Φ_(G) andΦ_(H), on the FSR bandwidth is so small that it is insignificant

The receiver 20,120 includes a microprocessor system for operating thereceiver 20,120 according to instructions stored in a memory. Theseinstructions include the above described bandwidth (FSR) controlalgorithm 33,133, the imbalance control algorithm 64,164 and the signalquality feedback 92,192. Signal quality for the receiver 20,120 may bedefined in terms of BER, ISI, eye opening ratio, and/or signal to noiseratio (SNR). Typically the minimum BER, the best compensation for ISI,the largest eye openings and the highest signal to noise ratios (SNR)sof the optical and electrical constructive and destructive path signalsare optimized, or nearly optimized, for the same selections andadjustments within the receiver 20,120. The algorithm 192 may operate ina feedback loop for minimizing BER.

FIG. 6A illustrates a delay line interferometer (DLI) 150A as anembodiment of the DLI 150. Elements associated with the DLI 150A thatare analogous to elements associated with the DLI 150 are denoted byappending the reference identification numbers with the letter “A”. TheDLI 150A includes structural elements for an input port 165A, a transfer(FSR) phase element 146A, a mechanism or oven 174A, a partiallyreflecting first mirror 202A, a second mirror 204A, a third mirror 208A,and constructive and destructive output ports 166A and 168A.

The structural elements of DLI 150A are disposed as follows. The inputoptical signal 24 illuminates the front side of the partially reflectingfirst mirror 202A. The first mirror 202A is set at an angle to the pathof the optical signal 24 so that part of the signal 24 is reflected as asignal 212A and part of the signal 24 is passed through as a signal214A. The signal 212A is reflected from the second mirror 204A as asignal 216A back to the front side of the first mirror 202A. The signal214A illuminates the element 146A and emerges after a fine tune phasedelay as a signal 218A. The signal 218A reflects from the third mirror208A as a signal 222A.

The signal 222A illuminates the element 146A and emerges after the phasedelay as a signal 224A. The signal 224A illuminates the back side of thefirst mirror 202A. Part of the signal 224A is reflected from the backside of the first mirror 202A to combine with part of the signal 216Apassed through the front side of the first mirror 202A for providing asignal 226A at the constructive output port 166A. Part of the signal224A passes through the back side of the first mirror 202A to combinewith part of the signal 216A reflected from the front side of the firstmirror 202A for providing a signal 228A at the destructive output port168A.

The elements of the DLI 150A split the input signal 24 into a first path232A and a second path 234A. The transit time of the first path 232A isthe sum of the transit times of the signals 212A and 216A. The transittime of the second path 234A is the sum of the transit times of thesignals 214A, 218A, 222A and 224A plus two times the phase delay of theelement 146A. It should be noted that the element 146B may beconstructed in first and second segments disposed in the first andsecond paths, respectively, and for providing a signal delay adjustmentthat is the difference between the signal delays of the two elementsegments. The difference between the first and second path transit timesis the differential transit time Y that is used for demodulation of theinput optical signal 24. The time Y is fine tuned by adjusting thesignal phase delay in the element 146A in order to adjust the FSR phaseof the DLI 150A for adjusting the transfer function phase of theconstructive and destructive transfer functions G(f) and H(f) (see FIG.2).

The material for the element 146A is selected to have an optical indexthat depends upon temperature. The FSR phase controller 137A provides acontrol signal to adjust the temperature of the oven 174A in order tofine tune the delay of the element 146A for centering the constructiveand destructive transfer functions G(f) and H(f) of the DLI 150A on theoptical carrier frequency of the input optical signal 24.

FIG. 6B illustrates a delay line interferometer (DLI) 150B as anembodiment of the DLI 150. Elements associated with the DLI 150B thatare analogous to elements associated with the DLI 150 are denoted byappending the reference identification numbers by the letter “B”. TheDLI 150B includes structural elements for an input port 165B, a transferFSR bandwidth element 148B, a transfer (FSR) phase element 146B, amechanism or oven 174B, a partially reflecting first mirror 202B, asecond mirror 204B, a third mirror 208B, and constructive anddestructive output ports 166B and 168B.

The structural elements of DLI 150B are disposed as follows. The inputoptical signal 24 illuminates the front side of the partially reflectingfirst mirror 202B. The first mirror 202B is set at an angle to the pathof the optical signal 24 so that part of the signal 24 is reflected as asignal 212B and part of the signal 24 is passed through as a signal214B. The signal 212B is reflected from the second mirror 204B as asignal 216B back to the front side of the first mirror 202B. The signal214B illuminates the element 148B and emerges after the delay Z as asignal 217B. The signal 217B illuminates the element 146B and emergesafter a fine tune phase delay as a signal 218B. The signal 218B reflectsfrom the third mirror 208B as a signal 222B.

The signal 222B illuminates the element 146B and emerges after the phasedelay as a signal 223B. The signal 223B illuminates the element 148B andemerges after the delay Z as a signal 224B. The signal 224B illuminatesthe back side of the first mirror 202B. Part of the signal 224B isreflected from the back side of the first mirror 202B to combine withpart of the signal 216B passed through the front side of the firstmirror 202B for providing a signal 226B at the constructive output port166B. Part of the signal 224B passes through the back side of the firstmirror 202B to combine with part of the signal 216B reflected from thefront side of the first mirror 202B for providing a signal 228B at thedestructive output port 168B.

The elements of the DLI 150B split the input signal 24 into a first path232B and a second path 234B. The transit time of the first path 232B isthe sum of the transit times of the signals 212B and 216B. The transittime of the second path 234B is the sum of the transit times of thesignals 214B, 217B, 218B, 222B, 223B and 224B plus two times the phasedelay of the element 146B plus two times the delay Z. It should be notedthat the element 146B may be constructed in first and second segmentsdisposed in the first and second paths, respectively, and for providinga signal delay adjustment that is the difference between the signaldelays of the two element segments. Similarly, the element 148B may beconstructed in first and second segments disposed in the first andsecond paths, respectively, and for providing a controlled FSR delaythat is the difference between the signal delays of the two elementsegments. The difference between the first and second path transit timesis the differential transit time Y that is used for demodulation of theinput optical signal 24. The FSR delay Z is a part of the transit timedifference Y. A bandwidth (FSR) control algorithm 133B (FIG. 10)provides a calculation or control signal for providing the time Y byselecting or adjusting the delay Z of the element 148B in order toselect or adjust the FSR and the bandwidths of the constructive anddestructive transfer functions G(f) and H(f) (FIG. 2) for the DLI 150B.

The material for the element 146B is selected to have an optical indexthat depends upon temperature. The FSR phase controller 137B provides acontrol signal to adjust the temperature of the oven 174B in order tofine tune the delay of the element 146B for centering the constructiveand destructive transfer functions G(f) and H(f) (FIG. 2) of the DLI150B on the optical carrier frequency of the input optical signal 24.

FIG. 6C illustrates a delay line interferometer (DLI) 150C as anembodiment of the DLI 150. Elements associated with the DLI 150C thatare analogous to elements associated with the DLI 150 are denoted byappending the reference identification numbers by the letter “C”. TheDLI 150C includes structural elements for an input port 165C, a combinedtransfer FSR bandwidth element and phase element 148C,146C, a mechanismor oven 174C, a partially reflecting first mirror 202C, a second mirror204C, a third mirror 208C, and constructive and destructive output ports166C and 168C.

The structural elements of DLI 150C are disposed as follows. The inputoptical signal 24 illuminates the front side of the partially reflectingfirst mirror 202C. The first mirror 202C is set at an angle to the pathof the optical signal 24 so that part of the signal 24 is reflected as asignal 212C and part of the signal 24 is passed through as a signal214C. The signal 212C is reflected from the second mirror 204C as asignal 216C back to the front side of the first mirror 202C. The signal214C illuminates the element 148C,146C and emerges after the delay Z andan adjustment by the fine tune phase delay as a signal 218C. The signal218C reflects from the third mirror 208C as a signal 222C.

The signal 222C illuminates the element 148C,146C and emerges after thedelay Z and an adjustment by the phase delay as a signal 224C. Thesignal 224C illuminates the back side of the first mirror 202C. Part ofthe signal 224C is reflected from the back side of the first mirror 202Cto combine with part of the signal 216C passed through the front side ofthe first mirror 202C for providing a signal 226C at the constructiveoutput port 166C. Part of the signal 224C passes through the back sideof the first mirror 202C to combine with part of the signal 216Creflected from the front side of the first mirror 202C for providing asignal 228C at the destructive output port 168C.

The elements of the DLI 150C split the input signal 24 into a first path232C and a second path 234C. The transit time of the first path 232C isthe sum of the transit times of the signals 212C and 216C. The transittime of the second path 234C is the sum of the transit times of thesignals 214C, 218C, 222C and 224C plus two times the delay Z with theadjustment of the phase delay of the element 148C,146C. It should benoted that the element 146C,148C may be constructed in first and secondsegments disposed in the first and second paths, respectively, and forproviding a signal delay adjustment that is the difference between thesignal delays of the two element segments and a controlled FSR delaythat is the difference between the FSR delays of the two elementsegments. The difference between the first and second path transit timesis the differential transit time Y that is used for demodulation of theinput optical signal 24. The FSR delay Z is a part of the transit timedifference Y. A bandwidth (FSR) control algorithm 133C (FIG. 10)provides a calculation or control signal for providing the time Y byselecting or adjusting the delay Z of the element 148C,146C in order toselect or adjust the FSR and the bandwidths of the constructive anddestructive transfer functions G(f) and H(f) (FIG. 2) for the DLI 150C.

The material for the element 148C,146C is selected to have an opticalindex that depends upon temperature. The FSR phase controller 137Cprovides a control signal to adjust the temperature of the oven 174C inorder to fine tune the phase delay of the element 146C for centering theconstructive and destructive transfer functions G(f) and H(f) (FIG. 2)of the DLI 150C on the optical carrier frequency of the input opticalsignal 24.

FIG. 7 is a simplified flow chart of a method for receiving adifferential phase shift keyed (DPSK) optical signal transmitted througha transmission link channel. One or any combination of these steps maybe stored on a tangible medium 300 in a computer-readable form asinstructions to a computer for carrying out the steps.

In a step 301 constructive and destructive transfer functions arecalculated, looked up in a table based on calculations, or activelytuned for minimizing the effect of intersymbol interference (ISI) forimproving signal quality. The transfer functions may be implemented byselecting a delay Z in a signal path of a delay line interferometer(DLI) in order to select the free spectral range (FSR) of the DLI. Thedelay Z contributes to a differential time Y, in general not equal to aDPSK symbol time T, for providing differential demodulation. The signalquality may be determined in terms of bit error ratio (BER) for outputdata. In a first embodiment the delay Z is selected by dynamicallyadjusting the delay Z with feedback from a signal quality measurement inorder to minimize the BER. In a second embodiment the delay Z isselected by trial and error in order to minimize a measured BER. In athird embodiment the delay Z is selected based upon a BER measurement onanother optical transmission link channel where the other channel isknown to have the same channel bandwidth. In a fourth embodiment thedelay Z is selected by calculating from a known channel or spectrumbandwidth. In a fifth embodiment the delay Z is selected from a tablehaving calculations based on channel bandwidth or spectrum forminimizing BER. The calculations for FSR are shown in the chart of FIG.10. Signal quality analysis and measurements other than BER, such asmeasurements of eye openings, may be used in place of, or to augment BERdetection for the selection, adjustment or control of the delay Z. Theuser should be aware that the receiver 20 may lose lock on the inputsignal 24 when a new FSR delay Z is selected.

In a step 302 an optical gain imbalance β between constructive anddestructive output port signals is selected (as described above for theFSR delay Z) to minimize the effect of ISI and to obtain the for bestsignal quality. The calculations for gain imbalance are shown in FIG.11. The signal quality may be determined as described above. Thetransfer functions are modified by selecting a gain imbalance β betweenthe signals in the constructive and destructive signal paths. The ISIand signal quality may be determined in terms of bit error ratio (BER)for output data. In a first embodiment the gain imbalance β is selectedby dynamically adjusting the gain imbalance β with feedback from asignal quality measurement in order to minimize the BER. In a secondembodiment the gain imbalance β is selected by trial and error in orderto minimize a measured BER. In a third embodiment the gain imbalance βis selected based upon a BER measurement on another optical transmissionlink channel where the other channel is known to have the same channelbandwidth. In a fourth embodiment gain imbalance β is selected bycalculating from a known channel or spectrum bandwidth. In a fifthembodiment the gain imbalance β is selected from a table havingcalculations based on channel bandwidth or spectrum for minimizing BER.Signal quality analysis and measurements other than BER, such asmeasurements of eye openings, may be used in place of, or to augment BERdetection for the selection, adjustment or control of the gain imbalanceβ. The user should be aware that the receiver 20 may lose lock on theinput signal 24 when a new gain imbalance β is selected.

In a step 303 the phase of the constructive and destructive transferfunctions is adjusted for maximizing the signal power difference betweenoptical constructive and destructive path signals. The transfer functionphases may be adjusted as FSR phases while the system is in operationfor providing output data without overly degrading the output data byfine tuning the delay of a signal delay element in a signal path in theDLI. Optionally, the FSR phase is further tuned for best signal quality.The FSR phase adjustment tunes the constructive and destructive transferfunctions relative to the carrier frequency of the input optical signal.

FIG. 8 is a flow chart of a method using a calculated FSR and acalculated gain imbalance for receiving a differential phase shift keyed(DPSK) optical signal transmitted through a transmission link channel.Any one or more of these steps may be stored on a tangible medium 310 ina computer-readable form as instructions that may be read by a computerfor carrying out the steps. The reader may refer to the descriptions ofthe system 10 and optical receivers 20 and 120 for further details ofthe following steps.

Either during design, test or installation in a step 320 a free spectralrange (FSR) of a delay line interferometer (DLI) is calculated based oncharacteristics, particularly the bandwidth of the link 16, for thetransmission system 10 for obtaining the best signal quality and/orlowest bit error ratio (BER). In a step 322 optical and/or electricalgain imbalances are calculated based on the FSR of the DLI, the symbolrate R, and the characteristics of the transmission system 10,particularly the bandwidth of the filters 26, for obtaining the bestsignal quality and/or lowest bit error ratio (BER).

In operation the receiver 20,120 receives the modulated input signal 24in a step 324. In a step 330 the DLI having the pre-calculated FSRdifferentially decodes the signal 24 and uses optical interference forseparating the signal into constructive and destructive signal paths. Ina step 332 the FSR phase is adjusted for tuning the FSR transferfunctions relative to the carrier of the signal 24. In a step 334 theoptical gain imbalance is applied to the signals in the constructive anddestructive signal paths for providing optical constructive anddestructive signal outputs.

The modulations of the signals at the optical constructive anddestructive signal outputs are detected and converted to electricalsignals in a step 336. In a step 338 the electrical gain imbalance isapplied to the signals in the constructive and destructive signal pathsfor providing electrical constructive and destructive signal outputs.

Power-related measurements are detected in a step 342 for the signals atthe constructive and destructive signal outputs. When the gain imbalanceis applied to the electrical signals, the electrical output signals aremeasured. When the gain imbalance is applied to the optical signals butnot the electrical signals, either the optical or the electrical outputsignals may be measured. In one embodiment, the gain is applied to theoptical signals and the power-related detections are measurements of theaverage photocurrents for converting the optical modulation toelectrical signals. In a step 344 a normalized difference between thepower-related measurements is applied to adjust the FSR phase for thestep 332. In a step 352 the electrical constructive and destructive pathsignals are combined by taking the difference of the signals. Thedifference is issued as a baseband signal. Finally, in a step 354 theinput data from the transmitter 12 is estimated from the baseband signalfor providing output data.

FIG. 9 is a flow chart of a dynamic method where the FSR and the gainimbalance are adjusted according to BER for receiving a differentialphase shift keyed (DPSK) optical signal transmitted through atransmission link channel while attempts are being made for transmittingdata through the system 10. Any one or more of these steps may be storedon a tangible medium 360 in a computer-readable form as instructionsthat may be read by a computer for carrying out the steps. The readermay refer to the descriptions of the system 10 and optical receivers 20and 120 for further details of the following steps. It should be notedthat the data may require several re-transmissions as the receiver20,120 is being adjusted.

The input signal 24 is received at the start in the step 324. In thestep 330 the DLI differentially decodes the signal 24 and uses opticalinterference for separating the signal into constructive and destructivesignal paths. In the step 332 the FSR phase is adjusted for tuning theFSR transfer functions relative to the carrier of the signal 24. For asymmetrical signal spectrum, the FSR phase is tuned for centering theFSR transfer functions to the carrier of the signal 24. In the step 334the optical gain imbalance is applied to the signals in the constructiveand destructive signal paths for providing optical constructive anddestructive signal outputs.

The modulations of the signals at the optical constructive anddestructive signal outputs are detected and converted to electricalsignals in the step 336. In the step 338 the electrical gain imbalanceis applied to the signals in the constructive and destructive signalpaths for providing electrical constructive and destructive signaloutputs.

Power-related measurements are detected in the step 342 for the signalsat the constructive and destructive signal outputs. When the gainimbalance is applied to the electrical signals, the electrical outputsignals are measured. When gain imbalance is applied to the opticalsignals but not the electrical signals, either the optical or theelectrical output signals may be measured. In one embodiment, the gainis applied to the optical signals and the power-related detections aremeasurements of the average photocurrents for converting the opticalmodulation to electrical signals. In the step 344 a normalizeddifference between the power-related measurements is applied to adjustthe FSR phase for the step 332. In a step 352 the electricalconstructive and destructive path signals are combined by taking thedifference of the signals. The difference is issued as a basebandsignal.

The difference between the signals from the constructive and destructiveelectrical outputs is determined in the step 352. for providing abaseband signal. In the step 354 the input data from the transmitter 12is estimated from the baseband signal for providing output data.

A signal quality determined from the optical or electrical signals, or abit error ratio (BER), is measured for the output data in a step 372. Ina step 374, feedback for the signal quality or BER is applied to adjustthe FSR used in the step 330. In a step 376 feedback for the signalquality is applied to adjust the optical and/or gain imbalance for thestep 334. And optionally, in a step 378 feedback for the signal qualityis applied to adjust the FSR phase for the step 332. The steps 330, 332and/or 334 may be iterated until no further improvement in signalquality is detected. Whenever the FSR is changed due to a new selectionor adjustment in the step 330, the FSR phase must be re-tuned in thestep 332.

FIG. 10 is an exemplary chart for the bandwidth (FSR) control algorithms33, 133 and 133A-C for calculating the optimum FSR for the DLI 150(FIGS. 4, 5 and 6A-C) based on the effective optical bandwidth of thesystem 10. The FSR and the bandwidth are normalized to the symbol rate R(the inverse of the symbol time T) of the system 10. It can be seen thatthe optimum FSR is at least 10% greater than the symbol rate R. It canalso be seen that the optimum FSR is at least 20% greater than thesymbol rate R when the effective optical bandwidth of the system 10 isless than the symbol rate R. It should be noted that the FSR/R levels of1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 and 2 are provided bydifferential demodulation transit times of about 90.9%, 83.3%, 76.9%,71.4%, 66.7%, 62.5%, 58.8%, 55.6%, 52.6% and 50%, respectively, of thesymbol time T for the modulated optical input signal 24.

FIG. 11 is an exemplary chart for the gain imbalance control algorithms64 and 164 for the calculating the extra gain imbalance to be applied bythe optical imbalancer 152 and/or the electrical imbalancer 156. Thegain imbalance term β is calculated from the FSR for the DLI 150, theeffective optical bandwidth of the system 10, and the symbol rate R ofthe system 10.

Referring back to FIG. 5, the optical receiver 120 provides controlmechanisms for adjusting both the FSR and the imbalance to optimize thedetection performance (e.g., by minimizing the BER). The optical delayline interferometer inside the signal processor 150 can be implementedto include an optical input port that receives a modulated optical inputsignal carrying a baseband signal, a first optical path that receives afirst part of the received modulated optical input signal, a secondoptical path that receives a second part of the received modulatedoptical input signal, and an optical output port that combines lightfrom the first and second optical paths to cause optical interferenceand produces a constructive optical output signal having a constructivetransfer function and a destructive optical output signal having adestructive transfer function. The first optical detector 182 convertsthe constructive optical output signal into a first electrical signaland the second optical detector 182 converts the destructive opticaloutput signal into a second electrical signal. A signal combiner 136combines the first and second electrical signals to produce a differencebetween the first and second electrical signals that represents arecovery of the baseband signal in the received modulated optical inputsignal. The data estimator 132 processes the difference between thefirst and second electrical signals to recover digital data bits fromthe difference produced by the signal combiner 136. An optical controlelement 146 or 148 is disposed in at least one of the first and thesecond optical paths in the optical delay line interferometer to controla transfer function bandwidth of the constructive transfer function andthe destructive transfer function. A first feedback circuit thatincludes the circuit 133 or 192 receives information on a signal qualityof the recovered digital data bits and produces a first feedback controlsignal to the optical control element 146 or 148 to set the transferfunction bandwidth in response to the signal quality to enhance thesignal quality. In combination with the above optical control element, asignal gain imbalance control unit (e.g., 176, 178, 186, 188) isprovided to control a difference between amplitudes of the first andsecond electrical signals at the signal combiner 136. A second feedbackcircuit which includes the imbalance control 164 receives theinformation on the signal quality of the recovered digital data bits andproduces a second feedback control signal to the signal gain imbalancecontrol unit in response to the signal quality to enhance the signalquality.

In the example in FIG. 5, the signal quality is measured by BER of thereceived data by using a BER detector 138 coupled downstream of the dataestimator 132. Alternatively, the threshold used by the data estimator132 for recovering the digital data bits can also be used to representthe signal quality. Under an optimal operating condition where thesignal quality is optimized, the threshold used by the data estimator132 is at zero. Therefore, the imbalance can be controlled by thefeedback to maintain the threshold of the data estimator 132 at zero.This control mechanism can also be used by the control unit 192 and theFSR bandwidth control 133.

In implementations, various control schemes can be used to control theimbalance via the imbalance control 164 in FIG. 5.

The imbalance control 164 in FIG. 5 can be a separate control modulecircuit or a control module that is integrated with other circuitry. Adesignated imbalance data memory can be used to store imbalance dataobtained during the manufacturing of the optical receiver.Alternatively, a memory unit that stores other data can be used to storethe imbalance data. At time of manufacture the nominal optical gainimbalance value is obtained and is stored in a calibration memory in theoptical receiver or one of the memory units mentioned above. During thereceiver initialization, the control processor in the optical receiverreads the optical gain imbalance value from the calibration memory andwrites this value to the imbalance control 164. FIG. 12 illustrates thisoperation. Based on this initial optical gain imbalance data, theimbalance control 164 sets the initial imbalance by controlling eitheror both of the electrical imbalancer 156 and the optical imbalancer 152in FIG. 5.

During the receiver initialization of the optical receiver in FIG. 5,the host system writes an optical filtering condition parameter(s) tothe control processor of the optical receiver. In one implementation, alookup table is provided to store imbalance values for the imbalancethat correspond to various optical filtering conditions. Duringinitialization of the optical receiver, a host system providesparameters of a current optical filtering condition under which themodulated optical input signal is to be transmitted to the opticalreceiver, and the lookup table is used to obtain a respective imbalancevalue for the current optical filtering condition. Next, the imbalanceof the optical receiver is set to the respective imbalance valueobtained from the lookup table. During normal operation of the opticalreceiver, the imbalance of the optical receiver is maintained at therespective imbalance value obtained from the lookup table. FIG. 13illustrates this operation. The control processor calculates the opticalgain imbalance setting from the lookup table of predetermined valuesthat are based on optical filtering conditions and writes this value tothe imbalance controller.

After the receiver initialization, the optical receiver is controlled tocontinuously update the imbalance control for optimizing the imbalance.FIG. 14 illustrates this continuous optimization operation. Theimbalance controller continuously reads the signal quality data andadjusts the optical gain imbalance to optimize the signal quality data.This process iterates throughout the normal operation of the receiver.

One mode of operation for the receiver in FIG. 5 is a one timeoptimization during the receiver initialization. While the opticalreceiver is initialized, the imbalance controller continuously reads thesignal quality data and adjusts the optical gain imbalance to optimizethe signal quality data. Once the optimum signal quality data point isreached, the imbalance controller stops adjusting the optical gainimbalance and optical gain imbalance is held at the optimum value duringnormal operation of the optical receiver. This operation is illustratedin FIG. 15. This process is repeated in the next receiverinitialization.

Another way for the imbalance control is to operate the imbalancecontroller to scan the imbalance gain across the operating range of theimbalance gain during the receiver initialization. The signal qualitydata is collected and monitored during this can. Upon completion of thisscan, the optical gain imbalance is set and held at the optimum valuedetermined during the scan where the signal quality is optimized. FIG.16 illustrates this operation.

The above described imbalance operations can be used for controlling theoptical imbalance by using one or both of the variable opticalattenuators 176 and 178 in the optical paths between the signalprocessor 150 and the two optical detectors 182 and 184, controlling theelectrical imbalance by using the electrical variable gain elements 186and 188 in the signal paths between the two optical detectors 182 and184 and the signal combiner 136, or controlling both optical imbalanceand electrical imbalance. The combination of the FSR control and theimbalance control allows optimization of the optical receiverperformance that can be difficult to achieve with other optical receiverdesigns.

While this document contains many specifics, these should not beconstrued as limitations on the scope of an invention or of what may beclaimed, but rather as descriptions of features specific to particularembodiments of the invention. Certain features that are described inthis document in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or a variation of a subcombination.

Only a few implementations are disclosed. However, variations andenhancements of the described implementations and other implementationscan be made based on what is described and illustrated in this document.

1. An optical receiver, comprising: an optical delay line interferometercomprising an optical input port that receives a modulated optical inputsignal carrying a baseband signal, a first optical path that receives afirst part of the received modulated optical input signal, a secondoptical path that receives a second part of the received modulatedoptical input signal, and an optical output port that combines lightfrom the first and second optical paths to cause optical interferenceand produces a constructive optical output signal having a constructivetransfer function and a destructive optical output signal having adestructive transfer function; a first optical detector that convertsthe constructive optical output signal into a first electrical signal; asecond optical detector that converts the destructive optical outputsignal into a second electrical signal; a signal combiner that combinesthe first and second electrical signals to produce a difference betweenthe first and second electrical signals that represents a recovery ofthe baseband signal in the received modulated optical input signal; adata estimator that processes the difference between the first andsecond electrical signals to recover digital data bits from thedifference produced by the signal combiner; an optical control elementdisposed in at least one of the first and the second optical paths inthe optical delay line interferometer to control a transfer functionbandwidth of the constructive transfer function and the destructivetransfer function; a first feedback circuit that receives information ona signal quality of the recovered digital data bits and produces a firstfeedback control signal to the optical control element to set thetransfer function bandwidth in response to the signal quality to enhancethe signal quality; a signal gain imbalance control unit that controls adifference between amplitudes of the first and second electrical signalsat the signal combiner; and a second feedback circuit that receives theinformation on the signal quality of the recovered digital data bits andproduces a second feedback control signal to the signal gain imbalancecontrol unit in response to the signal quality to enhance the signalquality.
 2. The receiver of claim 1, wherein: the signal gain imbalancecontrol unit comprises a variable optical attenuator in at least one oftwo optical paths carrying the constructive and destructive opticaloutput signals to the first and second optical detectors, respectively,to adjust an optical power level in response to the signal quality. 3.The receiver of claim 2, wherein: the signal gain imbalance control unitfurther comprises an electrical gain control circuit in at least one oftwo electrical paths carrying the first and second electrical signals tothe signal combiner, respectively, to adjust a signal amplitude inresponse to the signal quality.
 4. The receiver of claim 1, wherein: thesignal gain imbalance control unit comprises an electrical gain controlcircuit in at least one of two electrical paths carrying the first andsecond electrical signals to the signal combiner, respectively, toadjust a signal amplitude in response to the signal quality
 5. Thereceiver of claim 1, comprises: a signal quality detector connecteddownstream from the data estimator to measure the signal quality of therecovered digital data bits.
 6. The receiver of claim 5, wherein: thesignal quality detector comprises a bit error rate detector that detectsa bit error rate in the recovered digital data bits.
 7. The receiver ofclaim 1, wherein: the first and the second feedback circuits receive athreshold used by the data estimator for recovering the digital databits as an indicator of the signal quality and use the first and thefeedback control signals to control the optical control element and thesignal gain imbalance control unit to set the threshold to zero inenhancing the signal quality.
 8. The receiver of claim 1, wherein: thesecond feedback circuit continuously receives the information of thesignal quality of the recovered digital data bits and dynamicallyadjusts the signal gain imbalance control unit to continuously enhancethe signal quality.
 9. The receiver of claim 1, wherein: the receivedmodulated optical input signal has a modulation format based on a phaseshift keying (PSK) modulation.
 10. The receiver of claim 1, wherein: thereceived modulated optical input signal has a modulation format based ona differential phase shift keying (DPSK) modulation.
 11. The receiver ofclaim 1, wherein: the received modulated optical input signal has amodulation format based on a quadrature phase shift keying (QPSK)modulation or a differential QPSK modulation.
 12. The receiver of claim1, wherein: the received modulated optical input signal has a modulationformat based on an 8-phase shift keying modulation.
 13. The receiver ofclaim 1, wherein: the received modulated optical input signal has amodulation format based on a quadrature amplitude modulation (QAM)format.
 14. A method for using an optical receiver to extract digitaldata from a modulated optical input signal, comprising: operating anoptical delay line interferometer in an optical receiver to convert themodulated optical input signal based on optical interference into aconstructive optical output signal and a destructive optical outputsignal; operating a first optical detector in the optical receiver toconvert the constructive optical output signal into a first electricalsignal; operating a second optical detector in the optical receiver toconvert the destructive optical output signal into a second electricalsignal; using a difference between the first and second electricalsignals to recover a baseband signal carried by the received modulatedoptical input signal; measuring a signal quality of the recoveredbaseband signal; controlling the optical delay line interferometer tocontrol a transfer function bandwidth of the constructive anddestructive optical output signals to improve the measured signalquality; and controlling an imbalance between amplitudes of the firstand second electrical signals to improve the measured signal quality.15. The method of claim 14, wherein: during initialization of theoptical receiver, obtaining a predetermined gain imbalance value; andsetting the imbalance between amplitudes of the first and secondelectrical signals to the predetermined gain imbalance value; and duringnormal operation of the optical receiver, maintaining the imbalancebetween amplitudes of the first and second electrical signals at thepredetermined gain imbalance value.
 16. The method of claim 14,comprising: providing a lookup table of imbalance values for theimbalance between amplitudes of the first and second electrical signalsthat correspond to various optical filtering conditions for themodulated optical input signal; during initialization of the opticalreceiver, obtaining from a host system parameters of a current opticalfiltering condition under which the modulated optical input signal is tobe transmitted to the optical receiver; using the lookup table to obtaina respective imbalance value for the current optical filteringcondition; setting the imbalance between amplitudes of the first andsecond electrical signals to the respective imbalance value obtainedfrom the lookup table; and during normal operation of the opticalreceiver, maintaining the imbalance between amplitudes of the first andsecond electrical signals at the respective imbalance value obtainedfrom the lookup table.
 17. The method of claim 14, wherein: during theinitialization of the optical receiver, the signal quality of therecovered baseband signal is measured; and the imbalance betweenamplitudes of the first and second electrical signals is, accordingly,adjusted in response to the continuously measured signal quality tooptimize the measured signal quality, and the method further comprising:after the initialization of the optical receiver, setting the imbalancebetween amplitudes of the first and second electrical signals at aparticular imbalance value at the end of the initialization of theoptical receiver and maintaining the particular imbalance value duringnormal operation of the optical receiver.
 18. The method of claim 14,comprising: during the initialization of the optical receiver, operatingthe optical receiver to scan the imbalance between amplitudes of thefirst and second electrical signals over an operating range of values ofthe imbalance to perform measurements of the signal quality at thescanned imbalance values for the imbalance, selecting an imbalance valuefrom the scanning imbalance values that corresponds to the best measuredsignal quality, and: after the initialization of the optical receiver,setting the imbalance between amplitudes of the first and secondelectrical signals at the selected imbalance value at the end of theinitialization of the optical receiver and during normal operation ofthe optical receiver without adjustment during the normal operation ofthe optical receiver.
 19. The method of claim 14, wherein: the receivedmodulated optical input signal has a modulation format based on adifferential phase shift keying (DPSK) modulation.
 20. The method ofclaim 14, wherein: the received modulated optical input signal has amodulation format based on a quadrature phase shift keying (QPSK)modulation or a differential QPSK modulation.
 21. The method of claim14, wherein: the received modulated optical input signal has amodulation format based on an 8-phase shift keying modulation.
 22. Themethod of claim 14, wherein: the received modulated optical input signalhas a modulation format based on a quadrature amplitude modulation (QAM)format.
 23. A method for using an optical receiver to extract digitaldata from a modulated optical input signal, comprising: operating anoptical delay line interferometer in an optical receiver to convert themodulated optical input signal based on optical interference into aconstructive optical output signal and a destructive optical outputsignal; operating a first optical detector in the optical receiver toconvert the constructive optical output signal into a first electricalsignal; operating a second optical detector in the optical receiver toconvert the destructive optical output signal into a second electricalsignal; producing a difference between the first and second electricalsignals; operating a data estimator to process the difference to recoverdata of a baseband signal carried by the received modulated opticalinput signal; and using a threshold used by the data estimator forrecovering the data as an indicator of the signal quality to control animbalance between amplitudes of the first and second electrical signalsto set the threshold to zero during normal operation of the opticalreceiver.
 24. The method as in claim 23, comprising: providing themodulated optical input signal in a modulation format based on adifferential phase shift keying (DPSK) modulation, a quadrature phaseshift keying (QPSK) modulation, a differential QPSK modulation, an8-phase shift keying modulation, or a quadrature amplitude modulation(QAM) format